Illinois child support calculator

Illinois adopted the income-shares model in 2017. The idea: estimate what parents at this combined income would spend on their children in an intact household, then divide that obligation between them in proportion to income. The statute is 750 ILCS 5/505; the dollar amounts come from a schedule the Department of Healthcare and Family Services publishes and — since 2024 — revises every year.

The computation, in the statute's own order (§ 505(a)(1.5)): determine each parent's monthly net income; add them together; select the basic support obligation from the schedule for that combined income and number of children; and compute each parent's percentage share. The parent with the majority of parenting time is presumed to spend their share directly on the children; the other parent pays theirs as support.

"Net income" is the step with real choices in it. Illinois allows two methods: a standardized tax amount — single filer, standard deduction, FICA — read from the HFS Gross to Net Income Conversion Table, or an individualized amount based on actual taxes, which requires a joint stipulation or a court determination (§ 505(a)(3)(B)–(C)). That conversion table runs to many pages, differs by dependency exemptions, and is replaced annually — so this calculator takes each parent's monthly net income directly and leaves the gross-to-net step to the table or the official HFS estimator, which computes it from gross for you.

The 2026 Income Shares Schedule of Basic Obligations covers combined net incomes from $25.00 to $27,374.99 per month in $50 bands — 547 rows for one through six children, effective March 20, 2026. Because each row is a range, there is no rounding question about incomes that fall between rows (a genuine ambiguity in some states' tables — Florida's, for instance): every income up to the top of the schedule lands in exactly one band. The 2026 Addendum documents this year's changes, including extending the schedule's top from $26,724.99 to $27,374.99.

The full schedule is reproduced below, generated from the same data the calculator computes with — the chart and the engine cannot disagree.

When each parent has the children for at least 146 overnights a year (40% of the year), § 505(a)(3.8) switches to a different computation, on the theory that two full households cost more than one. The basic obligation is multiplied by 1.5; each parent's income share of that grossed-up figure is then multiplied by the other parent's percentage of time; and the two results are offset — the parent with the larger figure pays the difference. With the example parents ($4,200 and $2,800 net, two children) at 200 and 165 overnights, the grossed-up obligation is $2,953.50, the weighted shares come to $801.09 and $647.34, and the transfer falls to about $154 — against $788 under the standard worksheet. Crossing the 146th night is one of the largest single discontinuities in Illinois family law, which is why it is litigated so hard — and why SB 3524 lowers it to 110 nights in 2027.

The children's health insurance premium is allocated between the parents by income share (§ 505(a)(4)(D)). The court worksheet then settles it against the support obligation: if the paying parent provides the insurance, the receiving parent's share of the premium is subtracted from the payment; if the receiving parent provides it, the paying parent's share is added on top. Either way the insurer gets paid once and each parent bears their proportional share. Two statutory guardrails: a parent whose net income is below 133% of the federal poverty guideline ($1,768.90/month in 2026) generally cannot be ordered to contribute to private coverage, and reasonable cost is capped at 5% of the providing parent's gross income (§ 505(a)(4)(G)).

Child care is different in Illinois, and most calculators get this wrong: § 505(a)(3.7) makes work- and education-related child care a discretionary addition — "the court, in its discretion, may order" contribution, prorated by income share — and the official worksheet explicitly keeps it (and extraordinary school or extracurricular expenses under § 505(a)(3.6)) out of the support obligation line. This calculator does the same: enter a child care cost and it shows each parent's proportional share as its own line, not silently folded into the transfer.

At the bottom, § 505(a)(3.3a) creates a rebuttable presumption of a $40 per month per child minimum (capped at $120 total) for a paying parent whose gross income is at or below 75% of the federal poverty guideline — $997.50/month in 2026 ($15,960/year × 75% ÷ 12). Under § 505(a)(3.3b), a parent with no gross income who receives only means-tested assistance, or who cannot work due to disability, incarceration, or institutionalization, is presumed to owe a zero-dollar order. Because these rules key off gross income and this calculator takes net, it flags them rather than silently computing them. At the top, combined net income above $27,374.99 moves the case to court discretion under § 505(a)(3.5) — but the basic obligation may not be set below the schedule's highest row. This calculator shows that floor and labels it a floor.

Parent A nets $4,200 a month and has the children 265 nights; Parent B nets $2,800 and has them 100 nights; two children; Parent A carries the children's health insurance at $300 a month. Combined net income is $7,000, which lands in the $6,975.00–$7,024.99 band of the schedule: the basic obligation for two children is $1,969. Parent B's share of combined income is $2,800 ÷ $7,000 = 40%, so Parent B's support obligation is $1,969 × 40% = $787.60. Parent B has fewer overnights, so Parent B pays. The $300 premium is also split by income share: Parent B's 40% share is $120, and because the receiving parent provides the insurance, that $120 is added to the payment. Guideline transfer: $787.60 + $120.00 = $907.60 per month, about $10,891 a year. Every one of those steps appears, cited, in the calculator's "show the work" panel.